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Oral Tumors and Their Biology

Stephen Withrow United States

INCIDENCE AND RISK FACTORS

Oral cancer accounts for 6% of canine cancer and 3% of all cancers in cats. A general summary of the common oral tumors is found in Table 1.

PATHOLOGY AND NATURAL BEHAVIOR

Oral fibrosarcoma will often look surprisingly benign histologically. If the cancer in question is growing rapidly, is recurrent, or is invading bone, however, the clinician should dictate treatment as for malignant cancer. Fibrosarcoma is very invasive locally but metastasizes in less than 20% of cases (usually to the lungs).

Malignant melanoma can present a confusing histopathologic picture if the tumor or the biopsy section does not contain melanin (one-third of all cases). A his­topathologic diagnosis of “undifferentiated sarcoma” should be looked upon with suspicion for possible underlying melanoma. Melanoma has a predilection to metastasize to regional lymph nodes and then lung.

Squamous cell carcinoma is usually a straightforward histologic diagnosis. Severe and extensive involvement of bone is common in the cat. The metastatic rate in the cat is somewhat unknown since so few cats have their local disease controlled to observe the long-term metas­tatic potential. Metastasis in the canine is very site-dependent with the rostral oral cavity having a low metastatic rate and the caudal tongue and tonsil having a high metastatic potential.

The “traditional” epulides are similar to gingival hyper­plasia in appearance and are usually confined to one or two sites at the gum margin. They are slow growing, firm, and generally covered by intact epithelium. These are classified as fibrous epulides or ossifying epulides depending on the presence or absence of bone. A third class of epulides recently has been termed acanthomatous epulis instead of the previous term of ada­manti­noma. Some pathologists use the terms interchangeably. These are much more locally invasive and virtually always invade bone although they do not metastasize.

DIAGNOSTIC TECHNIQUES AND WORK-UP

The diagnostic evaluation for oral cancers is critical due to the wide ranges of cancer behavior and therapeutic options available. The most likely cancers to have positive chest radiographs at the time of diagnosis are mela­noma and squamous cell carcinoma of the caudal oral and pharyngeal area.

Cancers that are adherent to bone, other than simple epulides, should have regional radiographs taken under anesthesia. When 40% or more of the cortex is destroyed, lysis may be observed. However, apparently normal radiographs do not rule out bone invasion. Regional lymph nodes should be carefully palpated for enlargement or asymmetry. When palpable, they should be aspirated.

The last step, under the same anesthesia, is careful palpation and a large incisional biopsy. Large samples of healthy tissue at the edge and center of the lesion will increase the diagnostic yield. The biopsy site should be located in such a position as to be easily included in a possible resection. For small lesions, excisional biopsy may be undertaken at the time of initial evaluation. For more extensive disease, waiting for biopsy results to accurately plan treatment is encouraged.

PROGNOSIS

The prognosis for acanthomatous epulis/ada­mantinoma is excellent with surg­ery and/or irradiation with local recurrence rates less than 5%.

The outlook for squamous cell carcinoma is very site and species dependent. Cancers of the canine in the rostral mouth are curable with surgery or irradia­tion, while those of tonsil or base of the tongue, are highly metastatic and likely to recur locally or regionally. Local control of feline squamous cell carcinoma is poor with either surgery or radiation therapy.

Overall, over 25% of dogs with oral malignant melanomas will survive one year or more. Dogs with tumors less than two centimeters in diameter have a median survival of 511 days as opposed to dogs with lymph node involvement or tumors greater than two centimeters in diameter whose median survival is 164 days. Recurrent malignant mela­noma does worse than primarily treated disease that achieves permanent local control. Age, breed, sex, degree of pigmentation, microscopic appearance (?), and anatomic site are not prognos­tic.

Local control of fibrosarcoma is more of a problem than metastasis. The best one-year survivals with almost any treatment are no better than 25-40%. Fibro­sarcomas are generally considered radiation resistant for measurable disease but control rates are improved when treating microscopic postoperative residual disease.

TABLE 1: Summary of Common Oral Cancers of the Dog and Cat

 CANINE

 FELINE

Squamous cell carcinoma* (SCC)

Fibrosarcoma(FS)

Melanoma (MM)

Dental

Squamous cell carcinoma* (SCC)

Fibrosarcoma (FS)

Frequency (%)

20-30

10-20

30-40

5

70

20

Age (years)

10

7

12

9

10

10

Sex predilection

Equal

M > F

M > F

F > M

None

None

Patient size

Larger

Larger

Smaller

None

--

--

Site predilection

Rostral mandible

Palate

Buccal mucosa

Rostral mandible

Mandible or maxillary bone; tongue

Gingiva

Regional lymph node metastasis

Rare (except tonsil and tongue)

Rare

Common

Never

Rare

Rare

Distant metastasis

Rare (except tonsil and tongue)

Occasional

Common

Never

Rare

Occasional

Gross appearance

Red, cauliflower, raised, ulcerated

Flat, firm, ulcerated

2/3 pigmented, ulcerated

Like SCC

Proliferative in pharynx; minimal visible disease in oral cavity

Firm

% Bone involvement**

Variable

Common

Variable

Always

Common

Common

Radiation response***

Good

Poor-fair

Poor?****

Excellent

Poor

Poor

Surgery response

Good rostral; fair caudal

Fair—good (especially large lesions)

Fair to good

Excellent

Poor

Fair-good

Prognosis

Good—rostral; poor—caudal

Poor-fair

Poor-fair

Excellent

Very poor

Fair

Usual cause of death

Distant disease

Local disease

Distant disease

Rarely tumor related

Local disease

Local disease

Comments

Behavior varies dramatically from front (good) to back (poor) of oral cavity

Often looks low grade histologically but very invasive biologically

Presence or absence of pigment is not prognostic

May be confused with SCC histologically

Many tumors of mandible and maxilla have little or no visible oral disease but severe deep invasion of bone

*    Nontonsillar
**   Varies with site; if adherent to bone, must consider bone involved
***  Adjuvant microscopic postoperative residual disease is generally better than treating large volume macroscopic disease
**** Coarsely fractionated radiation (large dose/fraction) may achieve transient clinical response but is rarely curative


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